Evan H., Elenna
Yesterday we measured coherence in SR2 pitch damping to DARM ranging ~6-20 Hz (see green trace in first attachment). Evan and I tested this by adding a 12 Hz bandstop filter to the pitch damping loop and remeasuring coherence. The coherence went down at 12 Hz, so we concluded we needed to reduce the gain of the damping loop in teh 6-20 Hz region. Evan retuned the "ellip50" filter present in FM10 in the SR2 pitch damping filter bank to reduce the gain there, and then we reduce the overall loop gain by one half (2 to 1). We implemented these changes and saw no more coherence in that region (again see figure 1). However, this did not improve the SRC2 coherence to darm that we measured (figure 2). Also, we saw no improvement in DARM (figure 3) after the damping loop changes.
**Edit**
In my plots, all the references are before any damping changes, and all the live measurements are after. Therefore, the SR2 noise coherence to DARM has improved, however, I believe the SRC2 coherence to DARM has worsened.
I took additional coherence plots of SRC2 and AS_C before and after the change. In this plot, all the reference traces are before the SR2 pitch damping change, and all the live traces are after.
Old vs. new filter shown in the attachment. This filter was propagated to all HSTS M1 pitch damping loops, and the EPICS gain records (which were all different) were regularized. The changes were
- SUS-PRM_M1_DAMP_P_GAIN: was −5, now −1
- SUS-PR2_M1_DAMP_P_GAIN: was −2, now −1
- SUS-SRM_M1_DAMP_P_GAIN: was −3, now −1
- SUS-SR2_M1_DAMP_P_GAIN: was −2, now −1
The interferometer relocked fine and there appear to be no (new) ASC stability issues.
The HSTSs still have inconsistencies in their EPICS gains for some of the other 5 M1 DOFs. We should have an OLTF campaign to sensibly regularize everything.
